With the development of portable electrode devices such as cellular phones, notebook computers, and camcorders, studies on rechargeable secondary batteries are actively performed. Particularly, the lithium secondary battery operating at a voltage of approximately 3.6V has capacity more than three times that of nickel-cadmium battery or nickel-metal hydride battery, which is widely used as a power supply of portable electronic devices, and high energy density per unit weight, and thus utilization and study of the lithium secondary battery become rapidly activated.
The lithium secondary battery can be classified into a liquid electrolyte battery and a polymer electrolyte battery according to electrolyte type. Generally, a battery using a liquid electrolyte is referred to as lithium ion battery and a battery using a polymer electrolyte is referred to as a lithium polymer battery. The lithium secondary battery can be manufactured in various forms, typically, in cylindrical and square shapes. The lithium polymer battery, which is currently spotlighted, is manufactured in a deformable pouch shape such that its shape is freely changed. In addition, the lithium polymer battery is advantageous to reduce the thicknesses and weights of portable electronic devices since it light and has highs stability.
Recently, capacity increase and stability of the lithium secondary battery become important issues as the lithium secondary battery is used as a power supply of electric vehicles as well as various electrode devices and studies on the capacity increase and stability of the lithium secondary battery are actively performed.
To increase the capacity of the lithium secondary battery, unit cells manufactured in a pouch type are stacked and electrically connected to assemble a battery pack to thereby provide a lithium secondary battery having required capacity. The size of the battery pack cannot be limitlessly increased and light, compact and high-capacity battery packs are required.
Furthermore, since a battery pack having a specific unit area has electrically connected unit battery cells, the battery pack may explode if one of the unit battery cells is heated or short-circuited. This increases a demand for battery packs having improved stability.
The shape of a unit battery cell 10 included in a conventional battery pack is illustrated in FIG. 1.
Referring to FIG. 1, the unit battery cell 10 includes an electrode assembly 30 and a case 20 providing a space for accommodating the electrode assembly 30. The electrode assembly 30 is composed of a positive plate, a negative plate and a separated interposed between the positive plate and the negative plate. The electrode assembly 30 is wound into a shape of jelly-roll type with the positive plate, the separator and the negative plate sequentially arranged or composed of multiple electrodes laminated in a stack structure.
Furthermore, the electrode assembly 30 includes a cathode lead 60 and an anode lead 70 electrically connected to each pole plate and exposed to the outside of the sealed case 20. The case 20 is of pouch type that is composed of a thin metal film and an insulating film bonded to both sides of the thin metal film and can be freely bent, distinguished from a cylindrical or square type case formed from a thick metal plate, to thereby accomplish compact electronic devices.
FIG. 2 is a partial enlarged view showing the top portion of the inside of the case 20 of the secondary battery shown in FIG. 1, where cathode taps 40 are combined in a concentrated manner and connected to the cathode lead 60. FIG. 3 is a plan view of the secondary battery in an assembled state, shown in FIG. 1. Referring to FIGS. 2 and 3, the cathode taps 40 extended from a cathode current collector 41 and projected are combined by welding, for example, and connected to the cathode lead 60 in the form of a welded portion. The cathode taps 40 of the welded portion are bent in an approximately V shape, and thus a connecting portion of electrode taps and an electrode lead can be referred to as a V-forming part. The cathode lead 60 is sealed by the battery case 20 with the end 61, to which the cathode tap welded portion is connected, exposed to the outside of the case 20.
Since the cathode taps 40 are combined to form the welded portion, as described above, the top end of the inside of the battery case 20 is spaced apart from the top end (or bottom end) of the electrode assembly 30 by a predetermined distance. Accordingly, the distance L1 between the top end (or bottom end) of the electrode assembly 30 and the insulating film is considerably long and a dead space irrespective of battery capacity is generated around the welded portion of the cathode taps and the cathode lead. In addition, if gas is generated in the battery pack, the generated gas can easily remain in the dead space.
The conventional pouch type secondary battery is inefficient since many dead spaces are generated in the battery due to its structure and, even when external pressure applied to the secondary battery is increased to restrain generation of gas, it is difficult to restrain the generation of gas because a space where gas can remain exists inside the battery.